Early work relating to network-based wireless location systems is described in commonly assigned U.S. Pat. No. 5,327,144 “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the '144 patent are disclosed in commonly assigned U.S. Pat. No. 5,608,410 “System for Locating a Source of Bursty Transmissions.” Early art in the location of mobile devices via network-based Angle of Arrival (AoA) and Hybrids of AoA and TDOA include the commonly held U.S. Pat. No. 4,728,959; “Direction finding localization system”; U.S. Pat. Nos. 6,119,013 and 6,108,555 both entitled “Enhanced time difference localization system.”
Enhancements for network-based wireless location systems for CDMA systems can be found in the commonly held U.S. Pat. No. 7,340,259 “Robust, efficient, localization system”; U.S. Pat. No. 6,546,256 “Robust, efficient, location-related measurement”; and U.S. Pat. No. 6,047,192 “Robust, efficient, localization system.”
The ability of a CDMA-based system to maintain multiple legs between the mobile device and the network base station(s) is called “soft hand-off” (IS-95/IS-2000) or “soft-handover” (UMTS).
A soft handoff in a CDMA-based system occurs based on the beacon or pilot signal strength of several sets of base stations as measured by the mobile device (User Equipment (UMTS) or Mobile Station (IS-95/IS-2000)).
These sets are known in IS-95/IS-2000 variously as the active set, the neighbor set, the candidate set and the remaining set. In a UMTS system, the roughly corresponding sets of NodeB's are deemed the active, monitored and detected sets.
The active set is the set of base stations or NodeB's through which active communication is established. This definition of the active set applies to all aforementioned CDMA-based Wireless Communications System (WCNs).
In IS-95/IS—2000, the neighbor set is a set of base stations in proximity to the active base stations and includes base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication, but through which active communication is not yet established. The remaining set is a set of base stations that have mobile detectable pilots, but are not of sufficient quality or power to be included in any of the other three sets.
In UMTS, in addition to the active set, two other mutually exclusive sets are defined. The “monitored set” includes non-active set cells nevertheless known to the network. In UMTS, these cells are included by the UTRAN in broadcast “CELL_INFO_LIST”. “Detected set cells” are those cells detected by the mobile station (also called the User Equipment or UE), which are not known to the network. In UMTS, these cells are not found in the CELL_INFO_LIST or in the active set.
In CDMA (IS-95 and IS-2000), the active set members typically have higher measured pilot signals strengths relative to the neighbor, candidate set and remaining sets. The mobile uses these sets to help manage the handover/handoff process known as Mobile-Assisted Handover (MAHO). When communications between the network and mobile are initially established, a mobile communicates through radio signaling with a single base station, typically the base station with the highest received pilot power, but always a base station that meets the threshold for inclusion into the active set. During soft-handoff, the active set contains more than one base station. The mobile monitors the pilot signal strength of the base stations in the active set, the candidate set, the neighbor set and the remaining set. During handoff, when a pilot signal strength of a base station in the neighbor or remaining set reaches a defined threshold level, that base station is added to the candidate set and removed from the neighbor or remaining set by the mobile. When the mobile detects a relatively strong candidate pilot, the UE transmits a “Pilot Strength Measurement Message” (PSMM) to a Base Station Controller/Packet Control Unit (BSC/PCU) along with a request to add the base station of that pilot signal to the UE's active set. The PSMM report is evaluated by the BSC which coordinates the processing of a soft handoff with the base stations associated with the strong detected pilot signals.
In CDMA-based systems wireless communications systems, using the UMTS WCN as an example and source of nomenclature, the concept of a ‘serving cell’ has been replaced with one-way, two-way, three-way, etc. softhandoff handover (SHO) to take advantage of macrodiversity. In the downlink (NodeB-to-UE), macrodiversity is accomplished by combining, in the mobile's RAKE receiver, multiple copies of the downlink signal caused by either transmission from multiple antennas or by the multi-path corruption of the transmitted signal.
In the uplink direction, macrodiversity is accomplished through the use of multiple receive antennas collecting multiple copies of the UE transmitted signal. Since the UE transmitted signal is multi-path corrupted, multiple levels of signal combining can take place.
In all CDMA-based radio air interface wireless communications systems, detection of surrounding cell beacons is complicated by frequency re-use and the power control used to minimize the Near-Far effects.
The near-far problem is a classic co-channel interference (also called cross-talk) problem in cellular frequency reuse radio networks. The near-far problem arises from the fact that radio signals from transmitters closer to the receiver of interest are received with smaller radio path-loss attenuation than are signals from transmitters located further away. Therefore the strong signal from the nearby transmitter will mask the weak signal from the more distant transmitter.
In CDMA-based radio networks, the near-far co-channel interference is actively minimized using dynamic output power adjustment of the transmitters both in the uplink (UE-to-NodeB) and downlink (NodeB-to-UE) directions. With dynamic output power adjustment the closer transmitters (with less radio path loss) broadcast with less power so that the SNR for all transmitters at the serving receiver is roughly the same.
A network Wireless Location Services scenario may include hybrids with downlink and Satellite location techniques for a CDMA-based wireless communications network (WCN) such as the Universal Mobile Telephone System (UMTS). The UMTS WCN is fully specified by the 3rd Generation Partnership Project (3GPP) since December 1998.
Detailed descriptions of Radio messages, message elements, and parameters for UMTS can be found in technical specification document 3GPP TS 24.008 “Mobile radio interface Layer 3 specification; Core network protocols; Stage 3” and 3GPP TS 25.331 “Radio Resource Control (RRC); Protocol specification”
Detailed descriptions of the Wireless Location Systems standardized for UMTS are detailed in technical specification 3GPP TS 25.305 “User Equipment (UE) positioning in Universal Terrestrial Radio Access Network (UTRAN); Stage 2”. Details on handovers in the exemplary UMTS network can be found in 3GPP TS 23.009; “Handover Procedures”, 3GPP TR 25.832; “Manifestations of Handover and SRNS Relocation” and 3GPP TR 25.936; “Handovers for real time services from PS domain”. 
The ETSI and 3GPP defined term LMU (Location Mobile Unit) is functionally equivalent to the ANSI defined term Position Determining Unit (PDE) or to the Signal Collection System (SCS) term as used in the cited TruePosition Patents. In a network-based WLS, consisting of geographically distributed receivers (LMUs) either overlaid in or integral to the local Wireless Communications Network with central server(s), the Serving-Mobile Location Center (SMC) connects to the core communications network. The central server(s) communicate with the WCN for the purposes of obtaining location triggers and collecting location tasking information which in this case includes the Active Set of the mobile of interest.
Prior U-TDOA systems required that at least one receiver, deemed the reference LMU receiver, to successfully demodulate at least part of the signal from the mobile of interest.
In a CDMA-based WCN with soft-handover, more than one LMU may be able to fully or partially demodulate the signal from the mobile of interest. The resultant full or partial signal demodulations may, via soft-combining, be used to reconstruct a replica of the original transmission that is less degraded than the best replica that could be obtained from any of the individual demodulations. This reconstructed reference signal is then made available to all LMUs involved in the location for correlation processing. LMUs that participate in the demodulation process are called “demodulating LMUs” or “demod LMUs.” In addition to the demodulating LMU receivers, geographically neighboring or proximate LMUs (“cooperators” or “coop LMUs”) may be tasked to collect signals from the mobile of interest for correlation with the reference signal. These cooperators may be an LMU, an LMU sector, or multiple antennas serving the same LMU. The set of potential cooperating LMUs also includes the demodulating LMUs. A “demod sector” is one LMU sector that is tasked for demodulation. A “coop sector” is one LMU sector that is tasked for cooperation. The problem of identifying which LMU sectors to task for demodulation is related to, but separate from, the problem of identifying which LMU sectors to task for cooperation. Although the techniques described herein may solve both problems, they may also be used to solving either problem independently of the other. Thus, for example, in an embodiment in which the WLS is not required to collect a reference signal because the reference signal is provided to the WLS by the WCN, the techniques described herein can be used to identify which LMUs to task for cooperation. In such a case the LMUs identified as demod LMUs by these techniques would be used only for cooperation and not for demodulation. Not every sector or cell may have a LMU installed (e.g., a sparse network deployment, as described in TruePosition's U.S. patent application Ser. Nos. 11/736,950, 11/736,920, 11/736,868 and 11/736,902; all entitled “Sparsed U-TDOA Wireless Location Networks”). Both coop sectors and demod sectors are limited to those sectors and cells that have an associated LMU.
Thus “serving sector” may refer to the coverage area of the serving cell. The term “LMU sector” may be used for that portion of an LMU responsible for receiving and processing radio signals from one receive antenna if receive diversity is not in use, or from multiple antennas located in close proximity to one another and providing diversity coverage of the same area if receive diversity is in use.
For network-based wireless location systems operating in a CDMA-based wireless communications network, selection of a most nearly optimal group of uplink receivers for network-based wireless location is problematic due to the power-control inherent in such networks and the resulting near far problem.